Filters 

Many air purifiers clean the air by using a filter that captures any extraneous particles present in the atmosphere. These type of purifiers are often built in existing heating and cooling systems and may come in the form of screens at the opening of each vent throughout the home.

Typical materials that these filters are made from include cotton, fibreglass, and foam, which are relatively dense materials that limit the size of the particles that are able to pass through them.

Pleated filters further decrease the amount and size of particles that are able to pass through, though they may be more expensive.

HEPA  (High Efficiency Particulate Air)

These filters meet a Department of Energy standard for removing particles from the air. The filter can consist of any material as long as it meets this standard.

The ULPA  (Ultra-Low Penetration Air) standard is even stricter than HEPA, and the filters that meet this standard are used for correcting slight chemical imbalances in the air. Advanced ULPAs are used most often in the nuclear power industry.

Ionizing Purifiers 

These air purifiers use a method called corona discharge to create charged ions. The large particles in the air, like dust and other contaminants, pass through the corona discharge (a small electrical field) and become ionized. They slowly clump together, becoming heavier, and eventually settle out of the air.

Ozone Generators 

An ozone generator works a lot like the  ionizing purifier, but it is designed to alter molecules of oxygen and turn them into ozone instead of ions. 

Manufacturers of these devices claim that the ozone deodorizes and disinfects the air, though the evidence on the effects is sparse.

Adsorbents 

Many purifiers have an internal adsorbent material that eliminates odours, fumes and chemicals in the air. The most common adsorbent is activated charcoal , which is extremely porous and has many nooks and crannies that can capture molecules.

Humidifiers 

Central Air Conditioning 

Space Heaters 

Portable Air Conditioners 

Electric Fans